Pump and motor hydraulic system and control therefor



Sept 1, 1953 J. w. BRlDwELL. ET A1. 2,650,473

PUMP AND MOTOR HYDRAULIC SYSTEM AND CONTROL THEREF'OR Filed July 7, 1951 Amro/v M. @mf/e BYf/mff//ff M. pmu/m50 ATTURW Patented Sept. yl, 1953 PUMP AND MOTOR HYDRAULIC SYSTEM AND CONTROL THEREFOR John W. Bridwell, Morton M. Coker, and Herschel M. Williamson, Peoria., Ill., assignors to Caterpillar Tractor Co., Peoria, Ill., a corporation of California.

Application July 7, 1951, Serial N0. 235,652

This invention relates to a hydraulic control mechanism for directing fluid under pressure to one or more double-acting hydraulically actuated jacks.

The present invention will have many applications but is particularly adapted for use with, and Will be described herein in connection with a hydraulically actuated jack for controlling the movement of an earth-working implement such as a tractor-mounted bulldozer or the like.

A hydraulic circuit for controlling the movement of a tractor-mounted bulldozer or the like usually comprises one or more double-acting hydraulically actuated `jacks, a iluid reservoir, a pump, and a control valve for controlling the flow of iluid to and from the jacks. The control valve is usually provided with four operating positions, namely, neutral or hold, raise, lower, and a iioat position in which the working implement is free to move in response to external forces. One of the principal difliculties encountered in conventional hydraulic systems of this type is that under certain operating conditionsy the load on the working implement causes it to move faster than the rate at which the pump can supply fluid to the jacks. As a result the iiuid displaced from the jacks is returned to the supply tank at a greater rate than it is withdrawn by the pump thus causing excessive pressure which may result in damage to the hydraulic system as for instance bursting of the fluid supply tank. Another disadvantage in conventional hydraulic systems is that the speed at which the working implement is moved is limited by the pump output even when external forces are applied urging it in the desired direction. This is particularly undesirable during lowering of the bulldozer blade at which time it is desirable to bring the blade downwardly into its Working position in the shortest possible time. Although in conventional hydraulic circuits, it is possible -for the bulldozer to move ahead of pump output resulting in a partial vacuum in the pressure end of the jack so that even though the bulldozer is brought into the working position rapidly, down pressure on the blade cannot be exerted until the jack cylinder is iilled from the pump.

It is, therefore, an object of the present invention to provide an improved hydraulic system for controlling movement of an earth working implement or the like having means permitting fluid to flow directly from the supply tank into the power line receiving pump output when the pressure in the supply tank exceeds the pressure in the power line thus preventing excessive 3 Claims. (Cl. 60-52) pressure in the system as well as permitting more rapid movement of the implement under the influence of external forces. Other objects and advantages of this invention will be made apparent in the following description wherein reference is made to the accompanying drawing.

In the drawing, a hydraulic system embodying the present invention is illustrated schematically in its association with a double-acting hydraulically actuated jack for controlling the movement of an earthmoving bulldozer.

The hydraulic circuit shown in the drawing comprises a iiuid supply tank I0, a pump I I, and a control valve I2 for directing fluid under pressure' through power lines I3 and I4 to either end of a double-acting hydraulic jackl This jack is shown as comprising a cylinder I5 containing a reciprocal piston II carried on a piston rod Il. The cylinder is pivotally supported from a tractor or the like, a part of which is indicated at I8, While the piston rod is pivotally secured to a bulldozer blade I9. Although it is not shown, the bulldozer blade may be provided with a pair of push arms pivotally secured to the tractor in a manner to permit raising or lowering of the bulldozer in response to movement of the piston rod Il.

The control valve I2 is disposed Within the supply tank I0 and comprises a valve body 2I having an elongated bore 22 for the reception of a valve spool 23 adapted for a sliding movement therein. The valve spool is provided with a plurality of spaced collars adapted to cooperate with suitable recesses and passages in the control valve body to effect direction of fluid from the pump to either end of the jack cylinder through power line I3 or I4 and to return fluid displaced from the cylinder back to the supply tank. 'The valve spool 23 extends through a suitable opening provided in the tank and at its extending end is connected to suitable manual control means (not shown) for controlling the position of the valve spool. A spring illustrated at 26 is associated with the opposite end of the valve spool normally urging it toward its neutral or hold position.

Each of the power lines I3 and I4 communi- Cates directly with the interior of the tank below l the fluid level therein by Way of a passage 2l side of the valve through passages 32 communicating with a recess 33 provided in the valve. In practice the spring 3| exerts a relatively small force on the valve so that it can be readily opened when the pressure in the tank exceeds the pressure in the power line. Upon opening of the valve, fluid is free to flow from the tank to the power Vline through passage 21 without passing through the pump or control valve.

This arrangement of parts is particularly advantageous in the event that external forces acting on the bulldozer cause the jack to move at a greater rate than the pump can transfer fluid from one side of -the piston to the other. Under these conditions fluid is returned to the tank faster than it is removed by the pump'resulting in an increase in the pressure Ywithin the tank. Simultaneously a negative pressure is produced in the opposite end of the cylinder causing the make-up valve 29 to open allowing fluid in the tank to flow directly to the low pressure end of the cylinder without passing through the pump and control valve. In practice it has been found that this condition frequently occurs during lowering of the bulldozer from its raised position which is accomplished by positioning the valve spool as shown in the drawing.

In operation, as the bulldozer is being lowered, its weight urges the piston I6 downwardy in the cylinder so that the fluid beneath the piston is displaced through the power line I3 and flows into the valve body and thence into the tank II! by way of a discharge passage shown at 34. The exact course taken by the fluid through the valve body is immaterial to an understanding of the present invention and since the valve is of more or less conventional construction, the Voperation of which is well understood in the art, it need not be described in further detail. As the fluid tends to be returned to the tank I at a rate exceeding the capacity of the pump II, the pressure in the tank will become greater and sometimes excessive unless otherwise relieved. This high pressure in the tank is, with the present structure, relieved through the make-up valve 29 in the line I4 which permits the fluid to flow back into the jack above the piston. When the bulldozer is moving upwardly as it sometimes may under external forces which exceed the tendency of the pump to lift the bulldozer, the reverse of the above described operation takes place. That is, the fluid above the piston flows to the tank II) through the line I4, the valve and the discharge passage 34 and if pressure builds up within the tank, 'it will be relieved through the make-up valve 29 from which it may be passed by way of line I3 to the lower end of the jack cylinder I5.

We claim:

1. In a hydraulic circuit which comprises a double acting hydraulic jack, a fluid reservoir, a pump, fluid passage means for connecting the pump inlet with the reservoir, conduits connecting the pump outlet with opposite ends of the jack, and a control valve for selectively directing fluid under pressure from the pump through either of said conduits to one end of the jack while iiuid from the opposite end returns through the other conduit back through the valve and into the reservoir, a make-up valve communieating between the reservoir and one conduit at a point between the control valve and the jack to relieve fluid from the reservoir to the jack without owing through the control valve if the rate of fluid returning to the reservoir exceeds the output of the pump.

2. In a hydraulic circuit which comprises a double acting hydraulic jack, a fluid reservoir, a pump, fluid passage means for connecting the pump inlet with the reservoir, conduits connecting the pump outlet with opposite ends of thev jack, and a control valve for selectively directing VfluidV under pressure from the pump through either of said conduits to one end ofthe jack while fluid from the opposite end returns through the other conduit back through the Valve and into the reservoir, a make-up valve communicating between the reservior and each of the conduits at positions between the control valve and the jack to relieve fluid from the reservoir to the jack without flowing through the control valve if the rate of fluid returning to the reservoir through one conduit exceeds the output of the pump directed to the jack through the other conduit. I

3. In a hydraulic circuit which comprises a double acting hydraulic jack, a fluid reservoir, a pump, fluid passage means for connecting the pump inlet with the reservoir, conduits connecting the pump outlet with opposite ends of the jack, and a control valve for selectively directing fluid 'under pressure from the pump through either of said conduits to one end of the jack while fluid from the opposite end returns through the other conduit back through the valve and into the reservoir. a make-up valve between the reservoir and each of the conduits to relieve fluid from the reservoir to the jack if the rate of fluid returning to the reservoir through one conduit exceeds the output of the pump directed to the jack through the other conduit, said make-up valves being disposed within the reservoir below the liquid level and communicating with said conduits at points intermediate the control valve and the jack.

JOHN W. BRIDWELL. MORTON M. COKER.

HERSCHEL M. WILLIAMSON.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,157,240 Keel May 9, 1939 2,363,235 Ellinwood Nov. 21, 1944 2,543,989 Rockwell Mar. 6, 1951 

